EP0507174A1 - Process for preparing chromiumtrioxide-containing pigments - Google Patents

Abstract

The invention relates to a process for preparing chromate-free Cr2O3-containing pigments having a spinel, haematite or corundum structure.

Description

The present invention relates to a process for the preparation of chromate-free Cr₂O₃-containing pigments with spinel, hematite or corundum structure.

Cr₂O₃ is part of a large number of pigments of different crystal structures. The colors of these pigments span a wide range of visible light. Thus, in the spinel system (ZnMg) (Fe, Cr) ₂O Farb, shades from ocher to dark brown are obtained by varying the cations. In the (ZnCo) (Cr, Al) ₂O₄ system, color shades from light green to dark blue can be obtained. Black pigments are also achieved in the spinel system. The color is caused by the changing composition of the oxides of Cu, Cr, Fe, Mn, Co, Ni. The hematite system (Fe, Cr) ₂O₃ leads to different brown-black pigments depending on the composition.

These pigments are technically important for use in paint, plastic, enamel and ceramics.

Products made by afterglowing pure Cr₂O₃ or mixtures of Cr₂O₃ and 0.1 - 5% TiO₂ are used as green pigments that crystallize in the corundum lattice in special applications e.g. used in the plastics, paint and refractory sectors, where coarser primary particles are required.

All these Cr₂O₃-containing pigments are prepared by mixing the corresponding starting oxides, hydroxides or compounds which form oxides under the reaction conditions, wet or dry, annealing in the oven and then brought to the desired fineness by grinding (DE-C 2 338 012 and DE-C 2 320 806).

However, these processes have the disadvantage that kiln clinker which contains larger amounts of Cr (VI) is formed during the annealing. The reprocessing, i.e. Grinding of these pigments must therefore be followed by a washing of the pigments, the wash water containing Cr (VI) must be subjected to extensive reductive aftertreatment and the Cr³⁺ precipitated as Cr (OH) ₃ and separated off.

In addition, during the subsequent drying of the washed pigments or color bodies, a partial regression of chromate cannot be ruled out.

Chromium (VI) -free, Cr₂O₃-containing color bodies can be obtained according to DE-A 3 118 966 by the addition of antimony (III) oxide during annealing, but antimony (III) oxide is a physiologically questionable agent.

The object of this invention is therefore to provide a method which does not have the disadvantages described.

Surprisingly, it has now been found that the addition of boric and / or phosphoric acid to the raw mixture before the annealing prevents the formation of chromate. The pigments no longer have to be washed, so that they can also be subjected to simple dry grinding.

The present invention is therefore a process for the preparation of chromate-free Cr₂O₃-containing pigments with spinel, hematite or corundum structure, which is characterized in that a raw material mixture corresponding to the composition of the Cr₂O₃-containing pigments prior to annealing with boric acid and / or phosphoric acid or its anhydrides in an amount of 0.5-10% by weight (calculated as B₂O₃ or P₂O₅), based on the raw materials used as oxides, is added. This raw mixture is then processed further using the customarily known methods. Additions of boric acid and phosphoric acid above this range are also effective, but make little sense.

From Ullmann's Encyclopedia of Industrial Chemistry, Volume A5, 1986, page 554, the addition of boric acid as a mineralizer in the production of spinel pigments is known. When boric acid is used as the mineralizer, amounts below the levels claimed here are usually used. At the However, the use of such small amounts of boric acid does not produce chromium (VI) -free products in the case of chromium-containing spinels.

The invention is explained below by way of example, without any restriction being seen here.

Examples example 1

1 150 kg Cr₂O₃, 283 kg Fe₂O₃ and 600 kg CuO are suspended in 1 400 l water and mixed well. Then 30 kg of H₃BO₃ is added to this mixture. This raw mixture is annealed in a directly heated rotary tube at 1000 ° C. After the annealing, the clinker brick is ground wet. The furnace clinker is free of Cr (VI) so that the grinding suspension can be dried directly on a dryer. After drying, a Cr (VI) -free black pigment is obtained.

Example 2 (comparative example )

The procedure is as in Example 1, but without the addition of boric acid. An oven clinker with a Cr (VI) content of approx. 200 ppm is obtained. The wet-ground pigment suspension is washed free in a filter press Cr (VI). The wash water is collected separately and reduced to Cr³⁺ with FeSO₄. From this solution the Cr³⁺ is precipitated with NaOH as Cr (OH) ₃ and separated. Only now can the wash water be released as waste water.

Example 3

The procedure is as in Example 1, except that the same amount of phosphoric acid is used instead of boric acid. A Cr (VI) -free clinker is obtained which is ground up wet and dried on a belt dryer without washing. The finished pigment is free of Cr (VI).

Example 4

300 kg CoO, 640 kg Al (OH) ₃ and 300 kg Cr₂O₃ are dry mixed in a ploughshare mixer. A solution of 100 l of 85% phosphoric acid in 100 l of water is added to this powder mixture and homogenized again. The moist mixture is then annealed at 1200 ° C. The blue clinker is free of Cr (VI) and can be ground dry or wet.

Example 5

The procedure is as in Example 4, except that 30 kg of boric acid are added to the raw mixture instead of phosphoric acid. After the annealing, a chromate-free kiln clinker is obtained, which can be ground dry or wet.

Example 6 (comparative example )

The procedure is as in Example 4, but without the addition of phosphoric acid. The resulting brick clinker contains a lot of chromate, so that the chromate must be reduced as described in Example 2 and precipitated as Cr (OH) ₃.

Example 7

350 kg of zinc oxide, 350 kg of iron oxide yellow and 350 kg of Cr₂O₃ are mixed well with 1,200 l of water and 40 kg of boric acid in a ball mill. This suspension is placed in a rotary drum oven, dried at 300 ° C. and then annealed at 1,200 ° C. The red-brown brick is free of chromate. It can be worked up wet or dry to form a pigment; reoxidation to chromate does not occur.

Example 8

The procedure is as in Example 7, but without the addition of boric acid. A furnace clinker with a high chromate content is obtained. The chromate-containing wash water obtained must be subjected to a complex reductive precipitation aftertreatment as described in Example 2.

Example 9

650 kg FeO (OH), 350 kg Cr₂O₃ and 40 kg boric acid are mixed dry and this mixture is annealed at 1200 ° C. The result is a chromate-free, brown-black colored body.

The pigment can be ground dry or wet. In the case of wet grinding, there is no need for the complex reductive precipitation aftertreatment. No chromate is reproduced when drying the wet ground paint.

Example 10 (comparative example )

The procedure is as in Example 9, except that boric acid is not used. An oven clinker containing approximately 300 ppm chromate is obtained. When working up, the after-treatment of the wash water described above must be carried out again.

Example 11

650 kg of α-FeO (OH) and 250 kg of Cr₂O₃ are mixed with 1,000 l of water and 20 l of 85% phosphoric acid.

The suspension is placed in a rotary drum oven, dried at 150 ° C and then annealed at 1,150 ° C. A brown-black, chromate-free kiln clinker is obtained which is processed as in Example 9.

Example 12 (comparative example )

The procedure is as in Example 11, without the addition of phosphoric acid. A clinker brick with a high chromate content is obtained. The wash water obtained after the grinding of the colored body has to be subjected to extensive post-treatment, as already described.

Example 13

1,000 kg of Cr₂O₃ and 40 kg of boric acid are mixed dry in a ploughshare mixer and this mixture is annealed in a directly heated trominel furnace at 1,200 ° C. The dark green brick is free of chromate. The grinding can be carried out dry or wet. In the case of wet grinding, no reductive post-treatment of precipitation is necessary.

Example 14 (comparative example )

If the procedure is as described in Example 13, but without the addition of boric acid, an oven clinker containing chromate is obtained. The wash water obtained after the wet grinding must be subjected to an after-treatment as already described.

Example 15

1,500 kg Cr₂O₃ and 20 kg TiO₂ are mixed together with 1,500 l water and 50 kg boric acid in a ball mill. The mixture is placed in a rotary drum oven, dried at 300 ° C and then annealed at 1150 ° C. The dark green clinker is free of chromate and, as already described, can be worked up dry or wet.

Example 16 (comparative example )

If one proceeds as described in Example 15 without the addition of boric acid, products containing high chromate are obtained. The wash water obtained during the reprocessing must be reductively treated before it can be released as waste water.

Claims (4)

Process for the production of chromate-free Cr₂O₃-containing pigments with spinel, hematite or corundum structure by annealing one of the raw material mixtures corresponding to the pigment, characterized in that the raw material mixture before the annealing contains 0.5 to 10% by weight of boric acid and / or phosphoric acid, calculated as B₂O₃ or P₂O₅, is added. A method according to claim 1, characterized in that the pigments are mixed phase pigments. A method according to claim 1, characterized in that the pigments are afterglow Cr₂O₃ pigments with a corundum structure. A method according to claim 1, characterized in that the after-annealed Cr₂O₃ pigments have a TiO₂ content of 0.1 to 5 wt .-%, based on the pigment.